fix(aci): handle fake alert rule ids in AlertRuleDetector lookup

src/sentry/workflow_engine/endpoints/organization_alertrule_detector_index.py

@@ -16,6 +16,7 @@
     RESPONSE_UNAUTHORIZED,
 )
 from sentry.apidocs.parameters import GlobalParams
+from sentry.incidents.endpoints.serializers.utils import get_object_id_from_fake_id
 from sentry.models.organization import Organization
 from sentry.workflow_engine.endpoints.serializers.alertrule_detector_serializer import (
     AlertRuleDetectorSerializer,
@@ -24,6 +25,7 @@
     AlertRuleDetectorValidator,
 )
 from sentry.workflow_engine.models.alertrule_detector import AlertRuleDetector
+from sentry.workflow_engine.models.detector import Detector
 
 
 @region_silo_endpoint
@@ -69,7 +71,28 @@ def get(self, request: Request, organization: Organization) -> Response:
             queryset = queryset.filter(rule_id=rule_id)
 
         alert_rule_detector = queryset.first()
-        if not alert_rule_detector:
-            raise ResourceDoesNotExist
 
-        return Response(serialize(alert_rule_detector, request.user))
+        if alert_rule_detector:
+            return Response(serialize(alert_rule_detector, request.user))
+
+        # Fallback: if alert_rule_id was provided but no AlertRuleDetector was found,
+        # try looking up Detector directly using calculated detector_id
+        if alert_rule_id:
+            try:
+                calculated_detector_id = get_object_id_from_fake_id(int(alert_rule_id))
+                detector = Detector.objects.get(
+                    id=calculated_detector_id, project__organization=organization
+                )
+
+                if detector:
+                    return Response(
+                        {
+                            "detectorId": str(detector.id),
+                            "alertRuleId": str(alert_rule_id),
+                            "ruleId": None,
+                        }
+                    )
+            except (ValueError, Detector.DoesNotExist):
+                pass
+
+        raise ResourceDoesNotExist

tests/sentry/workflow_engine/endpoints/test_organization_alertrule_detector.py

@@ -1,4 +1,5 @@
 from sentry.api.serializers import serialize
+from sentry.incidents.endpoints.serializers.utils import get_fake_id_from_object_id
 from sentry.testutils.cases import APITestCase
 from sentry.testutils.silo import region_silo_test
 
@@ -89,3 +90,33 @@ def test_organization_isolation(self) -> None:
 
     def test_get_without_any_filter(self) -> None:
         self.get_error_response(self.organization.slug, status_code=400)
+
+    def test_fallback_with_fake_alert_rule_id(self) -> None:
+        """
+        Test that when an alert rule doesn't exist, the endpoint falls back to looking up
+        the Detector by subtracting 10^9 from the alert_rule_id.
+        """
+        # Create a detector with no AlertRuleDetector mapping
+        detector = self.create_detector(project=self.project)
+
+        # Calculate the fake alert_rule_id
+        fake_alert_rule_id = get_fake_id_from_object_id(detector.id)
+
+        # Query using the fake alert_rule_id
+        response = self.get_success_response(
+            self.organization.slug, alert_rule_id=str(fake_alert_rule_id)
+        )
+
+        # Should return a fake AlertRuleDetector response
+        assert response.data == {
+            "detectorId": str(detector.id),
+            "alertRuleId": str(fake_alert_rule_id),
+            "ruleId": None,
+        }
+
+    def test_fallback_with_nonexistent_detector(self) -> None:
+        # Use a fake alert_rule_id that won't map to any real detector
+        nonexistent_fake_id = get_fake_id_from_object_id(999999)
+        self.get_error_response(
+            self.organization.slug, alert_rule_id=str(nonexistent_fake_id), status_code=404
+        )